The present invention relates to improvements in a speaker system for generating and allowing selective sound directivity and attenuation in any of four peripheral directions, wherein a loudspeaker produces a difference in sound pressure, i.e., radiates sound, in a selected direction, while providing selected attentuation in any of the four peripheral directions.
Conventionally, an acoustic lens has been used to effect a selected directivity of a loudspeaker. FIG. 11 illustrates an example of one such loudspeaker.
Such a speaker system utilizing an acoustic lens is often used to overcome a deficiency wherein the directivity of the sound pressure radiated from a loudspeaker having a finite diameter becomes sharp with an increase in frequency, thereby causing a strong sound pressure to be concentrated in the front plane. This speaker system operates in a fashion similar to an optical concave lens, i.e., there is a converting of a plane wave of the sound wave into the spherical wave.
More particularly, the acoustic lens applied in a speaker system shown in FIG. 11 operates effectively to alter a shape of the wave front of the sound wave by partly extending the travelling path of the sound wave. This acoustic lens (suitable for analogy to an optical concave lens) is formed of a lamination of perforated thin plates. In this case, if the shape of the wave front at the entrance of the lens is planar (such as in a horn loudspeaker), then the acoustic lens has little or no effect on the shape of the sound wave propagating in the proximity of the central axis, but causes appreciable delay of any sound wave propagating along an axis which is distanced from the central axis since, during propagation, it is required to pass between the laminated plates by detouring. As a result, the wave front is curved close to a spherical wave, thereby resulting in a wider directivity.
In addition to wide directivity embodiments, speaker systems which radiates sound into only a particular narrow area (i.e., having a narrow directivity), but which radiates little or no sound into a remaining area, have been in demand. However, only a special type of loudspeaker such as a parametric loudspeaker using ultrasonics has been successful in obtaining a sufficient directivity.
As one example of a deficient narrow directivity approach, completely reversing the acoustic lens configuration shown in FIG. 11 would cause a spherical wave applied to the input (i.e., wide end) to attempt conversion to a plane wave. Such an arrangement would effect an acoustic "convergent lens" analogous to an optical convex lens, at least for a short distance along the central axis of the reversed acoustic lens. However, although the reversed acoustic lens is effective in converting the spherical wave into the plane wave, the plane wave can be produced only over an area approximating a front projection area of the reverse acoustic (convex) lens, i.e., the sound wave will diffuse in directivity in the vicinity of the convex lens immediately after it passes through the lens, thus failing to obtain the narrow directivity after all.
That is, while a reverse configuration of FIG. 11 is effective in converting the sound wave of the spherical wave into the plane wave or somewhat convergent wave so as to have a certain level of directivity, a desired narrow directivity for extended distances beyond the reversed acoustic lens cannot be implemented. For these reasons, the narrow directivity cannot be obtained by a method which is basically oriented to control the sound travelling path without causing the sound wave to diffuse in directions away from the central axis. It is further difficult to obtain a described directivity in any of loudspeaker, i.e., upper and lower, left and right, as is in the convex lens.